A compression-type refrigerating machine is generally constituted of a compressor, a condenser, an expansion valve and an evaporator, and has a structure in which a mixed fluid of a refrigerant and a lubricating oil is circulated through the closed system. Heretofore, in the compression-type refrigerator, dichlorofluoromethane (R12), chlorodifluoromethane (R22) or the like has mainly been used as the refrigerant, and various types of mineral oil and synthetic oil have been used as the lubricant.
However, chlorofluorohydrocarbons, such as R12 and R22 described above, are being more rigorously restricted world-wide because of a fear of their bringing about environmental pollution such as the ozonosphere destruction. By this reason, hydrogen-containing Flon compounds [a "Flon compound" means a chlorofluorocarbon, a hydrofluorocarbon, and a hydrochlorofluorocarbon in general] such as hydrofluorocarbons and hydrochlorofluorocarbons are attracting attention as the novel types of refrigerant. The hydrogen-containing fluorocarbons, particularly hydrofluorocarbons, typified by 1,1,1,2-tetrafluoroethane (Flon 134a), are preferred as the refrigerant for compression-type refrigerating machines because they are free from the possibility of causing the ozonosphere destruction and can replace Flon 12 with little modification in the structure of refrigerating machines which have heretofore been used.
It is known that the above-mentioned new alternative Flon-based refrigerant is different in properties from the conventional Flon-based refrigerant, and that a blend is useful as a refrigerating machine oil to be employed therewith, which blend comprises a base oil such as a polyalkylene glycol, polyester, polyol ester, polycarbonate and polyvinyl ether, and any of a variety of additives such as an antioxidant, extreme pressure agent, antifoam and hydrolysis inhibitor.
However, the aforesaid refrigerating machine oil sufferes a serious problem in practice that it is poor in lubricating performance in the atmosphere of the above-mentioned refrigerant and in particular, it unfavorably increases the wear between an aluminum material and a steel material in a refrigerating machine for an automobile air conditioner or an electrical refrigerator. The frictional part between the aluminum material and the steel material is an element of lubricative importance as it is used between a piston and a piston shoe, a swash plate and a shoe, etc. in a reciprocating type compressor (especially, a swash plate type), and between a vane and a housing, etc. in a rotary type compressor.
On the other hand there are known a variety of antiwear improvers, but it is the actual circumstance at the present time that there is still unknown a method capable of effectively preventing the wear between the aluminum material and a steel material without impairing lubricating stability in a special atmosphere of a Flon compound.